Income Taxes (Details) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2010
Income Taxes
Consolidated provision for income taxes	$ 13,714,000	$ 14,150,000	$ 32,484,000	$ 28,681,000	$ 41,296,000	$ 34,402,000
Consolidated effective federal and state income tax rates (as a percent)	36.50%	37.10%	36.70%	37.30%	36.70%	37.30%
Advanced coal investment tax credit									17,700,000
Maximum tax credit utilized							$ 9,000,000	$ 700,000